1. Technical Field
The present invention relates generally to material testing and more particularly to a method of recertifying the load on a bearing member via ultrasound techniques.
2. Discussion of the Related Art
The pulsed phase locked loop strain monitor, or P2L2, is a device which can be utilized to nondestructively measure the load in a bearing member such as a bolt, connector, etc. The P2L2, described in U.S. Pat. No. 4,363,242 to Heyman, measures acoustic phase changes and displays frequency changes which are indicative of changes in the load of the bearing member. By only determining load changes, the P2L2 requires that the ultrasonic sensor be kept on the bearing member during tightening or other load applications. Measurement of the load in a tightened bearing member, i.e., load recertification, is not possible because temperature variations as well as substitutions of various P2L2 elements such as transducers, cables, ultrasonic bonding, coupling, the P2L2 member itself, etc. are not corrected for a subsequent, after load measurement. In addition, it is often difficult to return to the same frequency point to correctly determine a subsequent frequency shift, thereby resulting in incorrect load determination.
Other non-destructive load testing systems include using an ultrasonic monitor to measure the time of flight of ultrasonic waves through a bolt. This measured time of flight is indicative of bolt loading. Such time of flight systems are harder to use and generally less accurate than P2L2 systems because the correct threshold trigger point must be selected to give the correct load measurement. In addition, load recertification of an already tightened bearing member is difficult for the reasons discussed above in connection with the P2L2 system.